1. Field of the Invention
The present invention relates to using two synthesizers for generating the RF and the IF oscillators in a radio, thereby making transceivers more robust with respect to interference.
2. Related Art
A transceiver includes both a receiver for receiving radio frequency (RF) signals and a transmitter for transmitting RF signals. The front-end of a receiver and the back-end of a transmitter can include similar components, such as a synthesizer for generating two output frequency signals. A brief overview of these components is described in FIG. 1, which illustrates a simplified receiver 100.
In receiver 100, a bandpass filter 102 receives the incoming signals from an antenna 101 and outputs a predetermined band of frequencies (while excluding those frequencies higher and lower than the predetermined band). A variable RF amplifier 103 can provide an initial amplification to that predetermined band of frequencies. A mixer 104 and IF filter 105 convert those amplified signals into intermediate frequency (IF) signals, which are then amplified by an IF amplifier 106.
At this point, mixers 107 and baseband filters 108 (including both I and Q branches) can generate signals in the desired channel, which are called the baseband signals. Baseband amplifiers 109 then amplify these baseband signals. Analog to digital converters (ADCs) 110 (provided for both the I and Q branches of baseband filters 108) transform the amplified baseband signals into digital signals that can be analyzed by a processing block 111.
In receiver 100, one synthesizer 120 generates two output frequency signals 121 and 122. Synthesizer 120 typically generates output frequency signal 122 by dividing output frequency signal 121 by two using a quadrature divider. U.S. Pat. No. 6,351,502, which issued on Feb. 26, 2002 to Atheros Communications, Inc., describes this process and is incorporated by reference herein. Mixer 104 receives output frequency signal 121. This signal has a frequency Frf, which could be greater or less than the frequency of the signal Fx1 output by RF amplifier 103. Mixer 104 mixes the signals having frequencies Fx1 and Frf and generates an output signal having two components. The first component, also called the intermediate component, is at a frequency Fx1-Frf (or Frf-Fx1, if Frf is greater than Fx1) (note that depending on the configuration, mixer 104 could be associated with the upper sideband such that frequency Fif (provided by mixer 107) is greater than frequency Frf). The second component is at a frequency Fx1+Frf. IF filter 105 receives this output signal and attenuates the second component at frequency Fx1+Frf, thereby leaving the intermediate component Fx1−Frf=Fx2. IF amplifier 106 amplifies the resulting signal having the intermediate component. In some embodiments, mixer 104 may be an image reject mixer, which may reduce the required performance of, or remove the need for, IF filter 105.
Mixers 107 receive output frequency signal 122, which has a frequency Fif. Therefore, mixers 107 generate output signals having two components. The first component is at a frequency Fx2-Fif. Generally, because Fx2 is substantially equal to Fif, the first component is called a baseband component whose frequency is substantially equal to 0. The second component is at a frequency Fx2+Fif (e.g. 2*Fif). Baseband filters 108 receive these output signals and attenuate the second component, thereby leaving the baseband component at a frequency of 0. Baseband amplifiers 109 amplify the resulting signals with the baseband component, thereby generating an output signal having a frequency Fbb (e.g. 0).
In the above-described frequency technique (also called a sliding IF), each RF channel has one corresponding voltage controlled oscillator (VCO) frequency and one intermediate frequency (IF). Typically, a design for receiver 100 would specify a fixed IF frequency and then adjust the RF frequency (i.e. by adjusting output frequency signal 121) so that irrespective of the current channel the desired IF frequency is output by output frequency signal 122.
Unfortunately, in receiver 100, interfering sources (e.g. fixed frequency clock waveforms) and noise (e.g. sidebands associated with a narrowband signals, called “birdies”) can couple into the RF frequency or the IF frequency. For example, FIG. 2 illustrates an RF passband 201, an IF passband 202, and a desired baseband (BB) passband 203. A signal in RF passband 201 is converted to a signal in IF passband 202 using a first output frequency signal (e.g. output frequency signal 121 of FIG. 1). Similarly, a signal in IF passband 202 can be converted to a signal in BB passband 203 using a second output frequency signal (e.g. output frequency signal 122).
As shown in FIG. 2, a large interfering source, called a “blocker” 204, can be present in IF passband 202. Unfortunately, blocker 204 cannot be filtered out because of its position relative to IF passband 202. That is, a blocker in or near IF passband 202 may be difficult or even impossible to filter without affecting the IF signal. Therefore, a mixer receiving an IF signal, blocker 204, and the signal from the second output frequency signal would undesirably generate another blocker, i.e. blocker 205, within baseband passband 203. This blocker propagation can significantly degrade receiver sensitivity and/or blocking performance. Note that in a transmitter a similar interfering source would cause a transmit spur.
Therefore, a need arises for a system and a technique that can advantageously avoid interference.